We propose a study, the Young Men's Affiliation Project (YMAP), to advance the utility of social network analysis by introducing a new way of addressing fundamental questions about social networks in relation to health-related behavior. We apply novel two-mode network methodologies to study the risk and health venue- based affiliation networks of younger men who have sex with men (YMSM) ages 16-29 and their relation to the transmission of human immunodeficiency virus (HIV) and sexually transmitted infections (STIs). The utility of social network analysis for understanding how infectious diseases are spread through risk networks based on sexual or drug use behaviors has been demonstrated. However, social network analysis has only rarely been utilized for investigating how affiliation with risk spaces, where HIV risk behavior can take place or health spaces, where HIV protective behaviors are promoted, contributes to disease transmission or health behavior more generally. Complex and potentially competing venue-based affiliation patterns, involving both risk and health venues may create networks with competing influences, some of which may contribute to risk and others to health; this dynamic is poorly understood, but reflects critical points for intervention. The purpose of YMAP is to investigate the HIV/STI risk and protective behaviors associated with complex webs of social networks created by YMSM's venue-based affiliations. YMAP will investigate and describe network influences on individuals' behavior (i.e., venue affiliation leads to behavior), as well as selective affiliation with venues based on behavioral homophily (i.e., YMSM with similar behaviors leads to venue affiliation). Our specific aims are to: 1) Identify venues where social interaction among YMSM occurs; create venue categories (i.e., risk, health, or both) considering their individual characteristics;2) Determine network influences on HIV/STI risk/protective behavior through risk and health venue affiliation and compare them by geographic location; 3) Investigate selective affiliation with venues based on similarity in HIV/STI risk and protective behaviors; and 4) Develop longitudinal two-mode network methodology to explore the dynamics of venue affiliation. The study will be implemented using a mixed method design, including qualitative field research, longitudinal quantitative data collection, and novel social network analyses. A racially and ethnically diverse sample of 700 YMSM will be recruited using respondent driven sampling in Houston and Chicago. YMAP will introduce new approaches to measuring affiliation-based network influence, structural centrality in affiliation networks, and estimating network configurations relating to behavior using two-mode exponential random graph modeling. Furthermore, the dynamics of affiliation networks in relation to HIV/STI risk/protective behaviors will be explored in two- wave of longitudinal data. YMAP responds directly to the PAR by addressing fundamental questions about social networks in relation to health and behavior, and to develop innovative methodologies to facilitate, improve, and expand the capability of social network analysis.